Low-voltage power breakers having a high rated current have, by virtue of their operation, a very wide contact system. This is dependent on the continuous current of the breaker and not on its switching capacity, which is generally the same for all breakers, independently of their rated current, and is approximately a maximum of 100 kA. The bar cross section and thus also the width of the busbars and of the contact system are therefore based on the continuous current.
With contact systems of this type, the moveable contacts are in the form of multiple-contact systems with a certain number of identical contact levers. These contact levers are conventionally provided not only with their main contact but also with a primary arcing contact and an arcing horn. This configuration causes the current to be commutated when the moveable switching contact is lifted off, and thus causes the current to be transferred from the main contact to the primary arcing contact. This results in the continuous current producing little heat, since the continuous current is passed through the main contacts, which interact with the stationary opposing contacts of the breaker and are not stressed due to arcs during switching. These main contacts do not erode and they therefore retain a good surface, for which reason their contact resistance and thus the increase in temperature owing to the continuous current are low.
With power breakers to be subjected to high and very high stress levels, for example with current-limiting power breakers, owing to the large number of contact levers provided and their pressure forces, considerable forces act on the contact support, in particular also on the mounting of the contact levers in the contact support, as a result of which, in some circumstances, the stress limits for the material are soon reached. It is therefore desirable to reduce the sum of the contact forces of all of the contact levers, arranged on a hinge pin, on the contact support. With the known, conventional low-voltage power breakers it is therefore usual to arrange a primary arcing contact on the stationary contact side, said primary arcing contact, owing to its dimensions, allowing only a certain number of contact levers of the moveable contact to come into contact with it. This stationary primary arcing contact is, in the case of conventional low-voltage power breakers, narrower than the main contact and is arranged centrally with respect to the entire contact width.
Such an arrangement is described, for example, in EP 0 410 902 B1. Here, a low-voltage power breaker having a moveable multiple contact for high rated currents is shown, which has two or more contact fingers of equal length which are arranged at a small distance from, and parallel to, one another, and a stationary main contact which interacts with a moveable main contact of each individual contact finger in the switched-on position. Furthermore, at least one moveable arcing contact is provided, and is arranged between the end of at least one contact finger and the moveable main contact.
These moveable arcing contacts interact with a stationary arcing contact which is designed such that the arc is centered in relation to the central axis of the switching pole. Thus, as can be seen in particular in FIG. 2 of the patent specification, it is central and is narrower than the stationary main contact. Other arrangements have moveable contacts where not all of the contact levers are provided with primary arcing contacts. In this case, these contact levers which have not been provided with primary arcing contacts are conventionally on the outsides of the multiple contact. An example of this is the low-voltage power breaker described in DE 197 27 696.
All of these arrangements have the disadvantage that the bending stress on the bearing bolt of the moveable contact levers is particularly high owing to the predominantly central action of forces of the contact levers.